Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. Displays using inorganic light emitting diodes (LEDs) are also in widespread use for outdoor signage and have been demonstrated in a 55-inch television.
Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Active-matrix circuits are commonly constructed with thin-film transistors (TFTs) in a semiconductor layer formed over a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, for example insulating dielectric layers and conductive metal layers are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, or wires.
Typically, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting diode (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single transistor to control each pixel. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Liquid crystals are readily controlled by a voltage applied to the single control transistor. In contrast, the light output from both organic and inorganic LEDs is a function of the current that passes through the LEDs. The light output by an LED is generally linear in response to current but is very non-linear in response to voltage. Thus, in order to provide a well-controlled LED, it is preferred to use a current-controlled circuit to drive each of the individual LEDs in a display. Furthermore, inorganic LEDs typically have variable efficiency at different current, voltage, or luminance levels. It is therefore more efficient to drive the inorganic LED with a particular desired constant current.
Pulse width modulation (PWM) schemes control luminance by varying the time during which a constant current is supplied to a light emitter. A fast response to a pulse is desirable to control the current and provide good temporal resolution for the light emitter. However, capacitance and inductance inherent in circuitry on a light-emitter substrate can reduce the frequency with which pulses can be applied to a light emitter. This problem is sometimes addresses by using pre-charge current pulses on the leading edge of the driving waveform and sometimes a discharge pulse on the trailing edge of the waveform. However, this increases power consumption in the system and can, for example, consume approximately half of the total power for controlling the light emitters.
Pulse-width modulation is used to provide dimming for light-emissive devices such as back-light units in liquid crystal displays. For example, U.S. Patent Publication No. 20080180381 describes a display apparatus with a PWM dimming control function in which the brightness of groups of LEDs in a backlight are controlled to provide local dimming and thereby improve the contrast of the LCD.
OLED displays are also known to include PWM control, for example as taught in U.S. Patent Publication No. 2011/0084993. In this design, a storage capacitor is used to store the data value desired for display at the pixel. A variable-length control signal for controlling a drive transistor with a constant current is formed by a difference between the analog data value and a triangular wave form. However, this design requires a large circuit and six control signals, limiting the display resolution for a thin-film transistor backplane.
U.S. Pat. No. 7,738,001 describes a passive-matrix control method for OLED displays. By comparing a data value to a counter a binary control signal indicates when the pixel should be turned on. This approach requires a counter and comparison circuit for each pixel in a row and is only feasible for passive-matrix displays. U.S. Pat. No. 5,731,802 describes a passive-matrix control method for displays. However, large passive-matrix displays suffer from flicker.
U.S. Pat. No. 5,912,712 discloses a method for expanding a pulse width modulation sequence to adapt to varying video frame times by controlling a clock signal. This design does not use pulse width modulation for controlling a display pixel.
There remains a need, therefore, for an active-matrix display system that provides an efficient, constant current drive signal to a light emitter and has a high resolution.